Organization and dynamics of the proteolipid complexes formed by annexin V and lipids in planar supported lipid bilayers

Annexin V Drives Stabilization of Damaged Asymmetric Phospholipid Bilayers

Annexins are soluble membrane-binding proteins that associate in a calcium dependent manner with anionic phospholipids. They play roles in membrane organization, signaling and vesicle transport and in several disease states including thrombosis and inflammation. Annexin V is believed to be involved in membrane repair. Mediated through binding to phosphatidylserine exposed at damaged plasma membrane, the protein forms crystalline networks that seal or stabilize small membrane tears. Herein, we model this biochemical mechanism to simulate membrane healing at microcavity array supported, transversally asymmetric, lipid bilayers (MSLBs) comprising 1,2-dioleoylsn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS). Varying annexin V concentration, lipid composition, and DOPS presence at each leaflet, fluorescence imaging and correlation spectroscopy confirmed that when DOPS was present at the external, annexin V, contacting leaflet, the protein assembled rapidly at the membrane interface to form a layer. From electrochemical impedance studies, the annexin layer decreased membrane capacitance while reducing resistance. With DOPS incorporated only at the lower (proximal) leaflet, no appreciable annexin assembly was observed over the first 21 h. This suggests that membrane asymmetry is preserved over this window and transversal diffusion of DOPS is slow. Intense laser light applied to the membrane, in which DOPS is initially isolated at the lower leaflet, was found to simulate membrane damage, stimulating the rapid assembly of annexin V at the membrane interface confirmed by fluorescence imaging, correlation spectroscopy, and electrochemical impedance measurements. The damage induced by light increased impedance and decreased membrane resistance. The resulting bilayer annexin V patched bilayer showed better temporal stability toward impedance changes when compared with that of the parent membrane. In summary, this simple model of annexin V assembly in a fluidic lipid membrane provides new insights into the assembly of annexins as well as an empirical basis for building patch-repair mechanisms into interfacial bilayer membrane assemblies.

Protein Diffusion on Charged Biopolymers: DNA versus Microtubule

Protein diffusion in lower-dimensional spaces is used for various cellular functions. For example, sliding on DNA is essential for proteins searching for their target sites, and protein diffusion on microtubules is important for proper cell division and neuronal development. On the one hand, these linear diffusion processes are mediated by long-range electrostatic interactions between positively charged proteins and negatively charged biopolymers and have similar characteristic diffusion coefficients. On the other hand, DNA and microtubules have different structural properties. Here, using computational approaches, we studied the mechanism of protein diffusion along DNA and microtubules by exploring the diffusion of both protein types on both biopolymers. We found that DNA-binding and microtubule-binding proteins can diffuse on each other's substrates; however, the adopted diffusion mechanism depends on the molecular properties of the diffusing proteins and the biopolymers. On the protein side, only DNA-binding proteins can perform rotation-coupled diffusion along DNA, with this being due to their higher net charge and its spatial organization at the DNA recognition helix. By contrast, the lower net charge on microtubule-binding proteins enables them to diffuse more quickly than DNA-binding proteins on both biopolymers. On the biopolymer side, microtubules possess intrinsically disordered, negatively charged C-terminal tails that interact with microtubule-binding proteins, thus supporting their diffusion. Thus, although both DNA-binding and microtubule-binding proteins can diffuse on the negatively charged biopolymers, the unique molecular features of the biopolymers and of their natural substrates are essential for function.

Annexin B12 Trimer Formation is Governed by a Network of Protein-Protein and Protein-Lipid Interactions

Membrane protein oligomerization mediates a wide range of biological events including signal transduction, viral infection and membrane curvature induction. However, the relative contributions of protein-protein and protein-membrane interactions to protein oligomerization remain poorly understood. Here, we used the Ca²⁺-dependent membrane-binding protein ANXB12 as a model system to determine the relative contributions of protein-protein and protein-membrane interactions toward trimer formation. Using an EPR-based detection method, we find that some protein-protein interactions are essential for trimer formation. Surprisingly, these interactions are largely hydrophobic, and they do not include the previously identified salt bridges, which are less important. Interfering with membrane interaction by mutating selected Ca²⁺-ligands or by introducing Lys residues in the membrane-binding loops had variable, strongly position-dependent effects on trimer formation. The strongest effect was observed for the E226Q/E105Q mutant, which almost fully abolished trimer formation without preventing membrane interaction. These results indicate that lipids engage in specific, trimer-stabilizing interactions that go beyond simply providing a concentration-enhancing surface. The finding that protein-membrane interactions are just as important as protein-protein interactions in ANXB12 trimer formation raises the possibility that the formation of specific lipid contacts could be a more widely used driving force for membrane-mediated oligomerization of proteins in general.

Quantitative analysis of annexin V-membrane interaction by flow cytometry

We constructed a green fluorescent phosphatidylserine (PS)-binding probe, which was generated by fusing enhanced green fluorescent protein (EGFP) to the C terminus of human annexin V (anxV). With this probe, we investigated anxV-membrane interaction under different calcium and anxV-EGFP concentrations through flow cytometry (FCM). A mathematical description of the binding characteristics is proposed and validated to quantify the relationship concerning the relative concentration of membrane-bound anxV (B), calcium concentration ([C]), and protein concentration ([P]). Further analyses reveal that [Formula: see text] is linear with [Formula: see text] or [Formula: see text] when [P] and [C] are fixed, respectively, which indicates that the anxV-membrane binding reaction may involve sequential multiple steps. Our study provides a reference for application of anxV in apoptosis detection. The mathematical expression facilitates exploration of the possible interactions between calcium, anxV, and membrane. The corresponding mathematical analysis strengthens the interpretation of the interaction data.

Electrostatically driven lipid-lysozyme mixed fibers display a multilamellar structure without amyloid features

Understanding the interactions between anionic lipid membranes and amyloidogenic proteins/peptides is key to elucidate the molecular mechanisms underlying the membrane-driven amyloid fiber formation. Here, hen egg-white lysozyme was used as a model protein to test whether this same process also occurs with non-amyloidogenic lipid-binding proteins/peptides. A complementary set of biophysical techniques was employed to study the structure and dynamics of the lipid-lysozyme mixed fibers produced at a low lipid/protein molar ratio that have been proposed earlier to present "amyloid-like" characteristics. The multilamellar architecture of these elongated mesoscopic structures was established by performing time-resolved Förster resonance energy transfer measurements, at both bulk (ensemble) and single-fiber level. The predominantly oligomeric lysozyme and phospholipids were both found to display significantly decreased lateral mobility when embedded in these mixed fibers. Notably, two-photon microscopy of Laurdan revealed that a pronounced membrane surface dehydration/increased molecular interfacial packing was produced exclusively in these elongated mixed supramolecular fibers present in the highly polymorphic samples. Infrared spectroscopic studies of lysozyme in these samples further showed that this protein did not exhibit a rich β-sheet structure characteristic of amyloid fibrils. These results support the conclusion that negatively charged lipid membranes do not have the general ability to trigger amyloid fibril formation of non-amyloidogenic proteins.

Validity and applicability of membrane model systems for studying interactions of peripheral membrane proteins with lipids

The cell membrane serves, at the same time, both as a barrier that segregates as well as a functional layer that facilitates selective communication. It is characterized as much by the complexity of its components as by the myriad of signaling process that it supports. And, herein lays the problems in its study and understanding of its behavior - it has a complex and dynamic nature that is further entangled by the fact that many events are both temporal and transient in their nature. Model membrane systems that bypass cellular complexity and compositional diversity have tremendously accelerated our understanding of the mechanisms and biological consequences of lipid-lipid and protein-lipid interactions. Concurrently, in some cases, the validity and applicability of model membrane systems are tarnished by inherent methodical limitations as well as undefined quality criteria. In this review we introduce membrane model systems widely used to study protein-lipid interactions in the context of key parameters of the membrane that govern lipid availability for peripheral membrane proteins. This article is part of a Special Issue entitled Tools to study lipid functions.

Quantitative analysis of self-association and mobility of annexin A4 at the plasma membrane

Annexins, found in most eukaryotic species, are cytosolic proteins that are able to bind negatively-charged phospholipids in a calcium-dependent manner. Annexin A4 (AnxA4) has been implicated in diverse cellular processes, including the regulation of exocytosis and ion-transport; however, its precise mechanistic role is not fully understood. AnxA4 has been shown to aggregate on lipid layers upon Ca(2+) binding in vitro, a characteristic that may be critical for its function. We have utilized advanced fluorescence microscopy to discern details on the mobility and self-assembly of AnxA4 after Ca(2+) influx at the plasma membrane in living cells. Total internal reflection microscopy in combination with Förster resonance energy transfer reveals that there is a delay between initial plasma membrane binding and the beginning of self-assembly and this process continues after the cytoplasmic pool has completely relocated. Number-and-brightness analysis suggests that the predominant membrane bound mobile form of the protein is trimeric. There also exists a pool of AnxA4 that forms highly immobile aggregates at the membrane. Fluorescence recovery after photobleaching suggests that the relative proportion of these two forms varies and is correlated with membrane morphology.

Assembly of membrane-bound protein complexes: detection and analysis by single molecule diffusion

Protein complexes assembled on membrane surfaces regulate a wide array of signaling pathways and cell processes. Thus, a molecular understanding of the membrane surface diffusion and regulatory events leading to the assembly of active membrane complexes is crucial to signaling biology and medicine. Here we present a novel single molecule diffusion analysis designed to detect complex formation on supported lipid bilayers. The usefulness of the method is illustrated by detection of an engineered, heterodimeric complex in which two membrane-bound pleckstrin homology (PH) domains associate stably, but reversibly, upon Ca(2+)-triggered binding of calmodulin (CaM) to a target peptide from myosin light chain kinase (MLCKp). Specifically, when a monomeric, fluorescent PH-CaM domain fusion protein diffusing on a supported bilayer binds a dark MLCKp-PH domain fusion protein, the heterodimeric complex is observed to diffuse nearly 2-fold more slowly than the monomer because both of its twin PH domains can simultaneously bind to the viscous bilayer. In a mixed population of monomers and heterodimers, the single molecule diffusion analysis resolves, identifies and quantitates the rapidly diffusing monomers and slowly diffusing heterodimers. The affinity of the CaM-MLCKp interaction is measured by titrating dark MLCKp-PH construct into the system, while monitoring the changing ratio of monomers and heterodimers, yielding a saturating binding curve. Strikingly, the apparent affinity of the CaM-MLCKp complex is ~10(2)-fold greater in the membrane system than in solution, apparently due to both faster complex association and slower complex dissociation on the membrane surface. More broadly, the present findings suggest that single molecule diffusion measurements on supported bilayers will provide an important tool for analyzing the 2D diffusion and assembly reactions governing the formation of diverse membrane-bound complexes, including key complexes from critical signaling pathways. The approach may also prove useful in pharmaceutical screening for compounds that inhibit membrane complex assembly or stability.

Find-me and eat-me signals in apoptotic cell clearance: progress and conundrums

Everyday we turnover billions of cells. The quick, efficient, and immunologically silent disposal of the dying cells requires a coordinated orchestration of multiple steps, through which phagocytes selectively recognize and engulf apoptotic cells. Recent studies have suggested an important role for soluble mediators released by apoptotic cells that attract phagocytes ("find-me" signals). New information has also emerged on multiple receptors that can recognize phosphatidylserine, the key "eat-me" signal exposed on the surface of apoptotic cells. This perspective discusses recent exciting progress, gaps in our understanding, and the conflicting issues that arise from the newly acquired knowledge.

Type I collagen-functionalized supported lipid bilayer as a cell culture platform

The supported phospholipid bilayer serves as an important biomimetic model for the cell membrane in both basic and applied scientific research. We have constructed a biomimetic platform based on a supported phospholipid bilayer that is functionalized with type I collagen to serve as a substrate for cell culture. To create the type I collagen-functionalized lipid bilayer assembly, a simple chemical approach was employed: lipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(glutaryl) (DP-NGPE), a carboxylic acid-functionalized phospholipid, were prepared and then fused onto an SiO(2) substrate to form a supported lipid bilayer. Subsequently, type I collagen molecules were introduced to form stable collagen-lipid conjugates via amide linkages with activated DP-NGPE lipids. The binding kinetics of the conjugation process and the resultant changes in film thickness and viscoelasticity were followed using the quartz crystal microbalance with dissipation (QCM-D) monitoring. The morphology of the conjugated collagen adlayer was investigated with atomic force microscopy (AFM). We observed that the adsorbed collagen molecules tended to self-assemble into fibrillar structures. Fluorescence recovery after photobleaching (FRAP) was utilized to estimate lateral lipid mobility, which was reduced by up to 20% after the coupling of type I collagen to the underlying lipid bilayer. As a cell culture platform, the collagen-conjugated supported lipid bilayer showed promising results. Smooth muscle cells (A10) retained normal growth behavior on the collagen-functionalized platform, unlike the bare POPC lipid bilayer and the POPC/DG-NGPE bilayer without collagen. The biomimetic functionalized lipid system presented here is a simple, yet effective approach for constructing a cell culture platform to explore the interactions between extracellular matrix components and cells.

Reversible immobilization of diffusive membrane-associated proteins using a liquid-gel bilayer phase transition: a case study of Annexin V monomers

We report a novel strategy for reversible immobilization of diffusive membrane-associated proteins in a native orientation using a liquid-gel bilayer phase transition, and its application to the single molecule study of Cy3-labeled Annexin V (A5) monomers on supported lipid bilayers containing phosphatidylserine (PS) in a Ca(2+)-rich environment. Total internal reflection fluorescence single molecule trajectory analysis revealed that, at low membrane occupancy, A5 monomers diffuse randomly on liquid phase bilayers and occasionally collide with other A5 monomers to form short-lived pseudodimers. During the liquid-to-gel bilayer phase transition, diffusive A5 monomers become immobilized mostly as isolated monomers, with some percentage of dimers and trimers. The EDTA-induced unbinding of immobilized Cy3-A5 spots indicates that Ca(2+)-bridges between A5 and PS lipids are preserved in the immobilized A5 monomers, confirming their native orientation on gel phase bilayers. Furthermore, the persistence of Ca(2+)-bridges during the liquid-to-gel phase transition, despite negligible A5 binding affinity to gel phase bilayers, strongly suggests the formation of tightly bound A5(Ca2+)m(PS)n complexes that diffuse and become immobilized as single units during the bilayer phase transition.

Double cushions preserve transmembrane protein mobility in supported bilayer systems

Supported lipid bilayers (SLBs) have been widely used as model systems to study cell membrane processes because they preserve the same 2D membrane fluidity found in living cells. One of the most significant limitations of this platform, however, is its inability to incorporate mobile transmembrane species. It is often postulated that transmembrane proteins reconstituted in SLBs lose their mobility because of direct interactions between the protein and the underlying substrate. Herein, we demonstrate a highly mobile fraction for a transmembrane protein, annexin V. Our strategy involves supporting the lipid bilayer on a double cushion, where we not only create a large space to accommodate the transmembrane portion of the macromolecule but also passivate the underlying substrate to reduce nonspecific protein-substrate interactions. The thickness of the confined water layer can be tuned by fusing vesicles containing polyethyleneglycol (PEG)-conjugated lipids of various molecular weights to a glass substrate that has first been passivated with a sacrificial layer of bovine serum albumin (BSA). The 2D fluidity of these systems was characterized by fluorescence recovery after photobleaching (FRAP) measurements. Uniform, mobile phospholipid bilayers with lipid diffusion coefficients of around 3 x 10(-8) cm2/s and percent mobile fractions of over 95% were obtained. Moreover, we obtained annexin V diffusion coefficients that were also around 3 x 10(-8) cm2/s with mobile fractions of up to 75%. This represents a significant improvement over bilayer platforms fabricated directly on glass or using single cushion strategies.

Biomimetic tethered lipid membranes designed for membrane-protein interaction studies

The complexity of the biological membranes restricts their direct investigation at the nanoscale. Lipid bilayer membranes have been developed as a model of biological membranes in order to allow the interaction and insertion of peptides and membrane proteins in a functional manner. Promising models have been developed in the past two decades and tethered bilayer design traduces constant improvement of membrane models. The formation of protein free solid tethered membranes can be achieved by direct vesicle fusion, Langmuir-Blodgett, Langmuir-Schaffer transfers, self assembly of various building blocks such as thiol on gold, silane on quartz, grafting of polymers, as well as ligand receptor recognition. In this review, the current state of different tethered bilayer membrane will be described. We will focus on critical analysis of the main advantages/drawbacks of each kind of model construction and their ability to allow protein incorporation in non-denaturing conditions. Some of the current drawbacks encountered in these biomimetic models can be overcome using an innovative tethered bilayer design based on a reliable and fast formation method. The successful protein incorporation of the Adenylate Cyclase produced by Bordetella pertussis and the voltage dependent anion channel (VDAC) was demonstrated on this model.

Immobilizing single lipid and channel molecules in artificial lipid bilayers with annexin A5

The effects of annexin A5 on the lateral diffusion of single-molecule lipids and single-molecule proteins were studied in an artificial lipid bilayer membrane. Annexin A5 is a member of the annexin superfamily, which binds preferentially to anionic phospholipids in a Ca2+-dependent manner. In this report, we were able to directly monitor single BODIPY 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (DHPE) and ryanodine receptor type 2 (RyR2) labeled with Cy5 molecules in lipid bilayers containing phosphatidylserine (PS) by using fluorescence microscopy. The diffusion coefficients were calculated at various annexin A5 concentrations. The diffusion coefficients of BODIPY-DHPE and Cy5-RyR2 in the absence of annexin A5 were 4.81 x 10(-8) cm(2)/s and 2.13 x 10(-8) cm(2)/s, respectively. In the presence of 1 microM annexin A5, the diffusion coefficients of BODIPY-DHPE and Cy5-RyR2 were 2.2 x 10(-10) cm(2)/s and 9.5 x 10(-11) cm(2)/s, respectively. Overall, 1 microM of annexin A5 was sufficient to induce a 200-fold decrease in the lateral diffusion coefficient. Additionally, we performed electrophysiological examinations and determined that annexin A5 has little effect on the function of RyR2. This means that annexin A5 can be used to immobilize RyR2 in a lipid bilayer when imaging and analyzing RyR2.

Asymmetric distribution of phosphatidyl serine in supported phospholipid bilayers on titanium dioxide

Supported phospholipid bilayers (SPBs) are useful for studying cell adhesion, cell-cell interactions, protein-lipid interactions, protein crystallization, and applications in biosensor and biomaterial areas. We have recently reported that SPBs could be formed on titanium dioxide, an important biomaterial, from vesicles containing anionic phospholipid phosphatidyl serine (PS) in the presence of calcium. Here, we show that the mobility of the fluorescently labeled PS present in these bilayers is severely restricted, whereas that of the zwitterionic phosphatidyl choline is not affected. Removal of calcium alleviated the restriction on the mobility of PS. Both components were found to be mobile in SPBs of identical compositions prepared in the presence of calcium on silica. To explain these results, we propose that, on TiO2, PS is trapped in the proximal leaflet of the bilayers. This proposal is supported by the results of protein adsorption experiments carried out on bilayers containing various amounts of PS prepared on silica and titania.

Stable and Fluid Ethylphosphocholine Membranes in a Poly(dimethylsiloxane) Microsensor for Toxin Detection in Flooded Waters

Highly stable and fluid supported bilayer membranes were fabricated by fusion of positively charged ethylphosphocholine (DOPC+) vesicles into poly(dimethylsiloxane) (PDMS) microchannels for immunosensing of cholera toxin (CT) in flooded waters. Compared to phosphatidylcholine (PC) layers in the microchannels, DOPC+ membranes show exceptionally strong resistance to air-dry damage, as demonstrated by fluorescence recovery after photobleaching (FRAP) measurements and protein adsorption studies. In FRAP experiments, the mobile fraction of PC membranes was found to decrease by 10% upon drying/rehydration and the lateral diffusion coefficient decreased from 2.2 to 1.6 microm(2)/s, whereas the mobile fraction and diffusion coefficient for DOPC+ membranes remain virtually unchanged during this process. Characterization by confocal microscopy reveals that only 1% of the DOPC+ membrane in the microchannels was removed by the drying/rehydration process, as compared to 11% for PC. Protein adsorption trends indicate that the charge of DOPC+ membranes allows for tuning of solution conditions to enable the desired protein-membrane interaction to predominate at the interface. A flow-based immunoassay for bacterial toxin was developed with 5% GM1/DOPC+ membranes in PDMS channels, and a detection limit of 250 amol for CT was obtained from the calibration curves. The assay was successfully applied to detection of CT spiked in water samples from the Santa Ana River, with nearly identical response and sensitivity.

Interaction of poly(L-lysine)-g-poly(ethylene glycol) with supported phospholipid bilayers

Interactions between the graft copolymer poly(L-lysine)-g-poly(ethylene glycol), PLL-g-PEG, and two kinds of surface-supported lipidic systems (supported phospholipid bilayers and supported vesicular layers) were investigated by a combination of microscopic and spectroscopic techniques. It was found that the application of the copolymer to zwitterionic or negatively charged supported bilayers in a buffer of low ionic strength led to their decomposition, with the resulting formation of free copolymer-lipid complexes. The same copolymer had no destructive effect on a supported vesicular layer made up of vesicles of identical composition. A comparison between poly(L-lysine), which did not induce decomposition of supported bilayers, and PLL-g-PEG copolymers with various amounts of PEG side chains per backbone lysine unit, suggested that steric repulsion between the PEG chains that developed upon adsorption of the polymer to the nearly planar surface of a supported phospholipid bilayer (SPB) was one of the factors responsible for the destruction of the SPBs by the copolymer. Other factors included the ionic strength of the buffer used and the quality of the bilayers, pointing toward the important role defects present in the SPBs play in the decomposition process.

A phospholipid bilayer supported under a polymerized Langmuir film

Phospholipid single bilayers supported on a hydrophilic solid substrate are extensively used in the study of the interaction between model membranes and proteins or polypeptides. In this article, the formation of a single dimyristoylphosphatidylcholine (DMPC) bilayer under an octadecyltrimethoxysilane (OTMS) polymerized Langmuir monolayer at the air-water interface is followed by Brewster angle microscopy (BAM) and polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). The formation of the bilayer is initiated by injection of dimyristoylphosphatidylcholine small unilamellar vesicles into the aqueous subphase. Brewster angle microscopy allows visualization of the kinetics of formation and the homogeneity of the bilayer. Spectral simulations of the polarization-modulated infrared reflection absorption spectroscopy spectra reveal that the bilayer thickness is 39 +/- 5 A. This system constitutes the first example of a phospholipid bilayer on a "nanoscopic" support and opens the way to studies involving supported bilayers using powerful experimental techniques such as x-ray reflectivity, vibrational spectroscopies, or Brewster angle microscopy.

Measurement of the affinity and cooperativity of annexin V–membrane binding under conditions of low membrane occupancy

We developed a method for measuring the binding affinity of annexin V for phospholipid vesicles and cells at very low levels of membrane occupancy. The annexin V-117 mutant was labeled with fluorescein iodoacetamide on its single N-terminal cysteine residue; binding to phospholipid vesicles containing phosphatidylserine (PS) and 2% rhodamine-phosphatidylethanolamine was measured by fluorescence quenching due to resonance energy transfer; binding to cells with exposed PS was measured by fluorometry after elution of bound protein. The equilibrium constant was calculated as a function of the midpoint of the calcium titration curve, the Hill coefficient, and the concentration of membrane binding sites. Calcium titrations at very low ratios of protein to membrane revealed Hill coefficients of approximately 8 for both vesicles and cells, far higher than previously measured, but as the protein-membrane ratio was increased above 3% of maximum membrane occupancy, the value of the Hill coefficient progressively decreased to a limiting value of about 2. High Hill coefficients were also observed for measurements performed at different ionic strengths and with membrane PS content varied over the range from 20 to 50%. This method allows the accurate determination of the affinity and cooperativity of annexin V-membrane binding and will be useful for the evaluation of modified annexin V derivatives intended for diagnostic and therapeutic applications.

EPR studies of phospholipid bilayers after lipoperoxidation

The molecular order of fatty acyl chains in oriented lipid bilayers on solid support (SPB), made of either natural or synthetic phospholipids oxidized by Fenton reagent and probed with spin labeled lecithin (5-DSPC) was studied by means of EPR spectrometry. Phospholipids (ASPC, EYPC, mitochondrial extract) were oxidized as either aqueous buffer/methanol dispersions or reverse-phase evaporation vesicles (REV) suspensions. Oxidation was preliminarily revealed both by assaying MDA and by detecting conjugated dienes. Oxidized phospholipid species was quantified by preparative TLC. The degree of order in oriented lipid bilayers of samples containing oxidized phospholipids was estimated by the loss of EPR spectral anisotropy, and an empirical index of the related bilayer disorder was calculated from the second derivative spectra. Bilayers made of each non-oxidized phospholipid species from either ethanol solutions or REV suspensions showed the highest anisotropy, while the increasing presence of oxidized lipids in the samples resulted in progressive loss of EPR spectral anisotropy. In contrast, vesicles containing 40% of the oxidized species maintained an unaltered fluidity gradient, while REV formation was hindered by oxidized phospholipid percentages higher than 45% for ASPC and EYPC, and 35% for Mitochondrial lipids (MtL). It is concluded that the early stages of lipoperoxidation bring about disordering of the phospholipid bilayer interior rather than fluidity alterations, and that prolonged oxidation may result in loss of structural and chemical properties of the bilayer until the structure no longer holds.

HDL-mediated cholesterol uptake and targeting to lipid droplets in adipocytes

Adipocytes express high levels of the HDL scavenger receptor class B type I in a differentiation-dependent manner. We thus have analyzed the routes of HDL cholesterol trafficking at different phases of adipocyte differentiation in the 3T3-L1 cell line. One novel and salient feature of this paper is the observation of a widespread distribution in the cell cytoplasm of Golgi markers, caveolin-2, and a fluorescent cholesterol analog NBD-cholesterol (NBD-chol), observed in the early phases of adipocyte formation, clearly distinct from that observed in mature fat cells (i.e., with fully formed lipid vesicles). Thus, in cells without visible lipid droplets, Golgi markers (Golgi 58K, Golgin 97, trans-Golgi network 38, Rab 6, and BODIPY-ceramide), caveolin-2, and NBD-chol all colocalize in a widespread distribution in the cell. In contrast, when lipid droplets are fully formed at latter stages, these markers clearly are distributed to distinct cell compartments: a compact juxtanuclear structure for the Golgi markers and caveolin-2, while NDB-chol concentrates in lipid droplets. In addition, disorganization of the Golgi using three different agents (Brefeldin, monensin, and N-ethyl-maleimide) drastically reduces NBD-chol uptake at different phases of adipocyte formation, strongly suggesting that the Golgi apparatus plays a critical role in HDL-mediated NBD uptake and routing to lipid droplets.

Macrophage recognition of externalized phosphatidylserine and phagocytosis of apoptotic Jurkat cells--existence of a threshold

Phosphatidylserine (PS) is predominantly confined to the inner leaflet of plasma membrane in cells, but it is externalized on the cell surface during apoptosis. This externalized PS is required for effective phagocytosis of apoptotic cells by macrophages. Because PS trans-bilayer asymmetry is not absolute in different types of nonapoptotic cells, we hypothesized that the amounts of externalized PS may be critical for macrophage discrimination between apoptotic and nonapoptotic cells. We developed a sensitive electron paramagnetic resonance method to quantify the amounts of externalized PS based on specific binding of paramagnetic annexin V-microbead conjugates with PS on cell surfaces. Using this technique, we found that nonapoptotic Jurkat cells externalize 0.9 pmol of endogenous PS/10(6) Jurkat cells. For cells with different amounts of integrated exogenous PS on their surface, no phagocytic response was observed at PS levels <5 pmol/10(6) Jurkat cells; at higher PS concentrations, phagocytosis increased in a concentration-dependent manner. Apoptosis in Jurkat cells caused externalization of approximately 240 pmol PS/10(6) Jurkat cells; these amounts of externalized PS are manyfold higher than the threshold amounts of PS required for phagocytosis. Thus, macrophages have a sensitivity threshold for PS externalized on the cell surface that provides for reliable recognition and distinction between normal cells with low contents of externalized PS and apoptotic cells with remarkably elevated PS levels.

Ca(2+) and membrane binding to annexin 3 modulate the structure and dynamics of its N terminus and domain III

Annexin 3 (ANX A3) represents approximately 1% of the total protein of human neutrophils and promotes tight contact between membranes of isolated specific granules in vitro leading to their aggregation. Like for other annexins, the primary molecular events of the action of this protein is likely its binding to negatively charged phospholipid membranes in a Ca(2+)-dependent manner, via Ca(2+)-binding sites located on the convex side of the highly conserved core of the molecule. The conformation and dynamics of domain III can be affected by this process, as it was shown for other members of the family. The 20 amino-acid, N-terminal segment of the protein also could be affected and also might play a role in the modulation of its binding to the membranes. The structure and dynamics of these two regions were investigated by fluorescence of the two tryptophan residues of the protein (respectively, W190 in domain III and W5 in the N-terminal segment) in the wild type and in single-tryptophan mutants. By contrast to ANX A5, which shows a closed conformation and a buried W187 residue in the absence of Ca(2+), domain III of ANX A3 exhibits an open conformation and a widely solvent-accessible W190 residue in the same conditions. This is in agreement with the three-dimensional structure of the ANX A3-E231A mutant lacking the bidentate Ca(2+) ligand in domain III. Ca(2+) in the millimolar concentration range provokes nevertheless a large mobility increase of the W190 residue, while interaction with the membranes reduces it slightly. In the N-terminal region, the W5 residue, inserted in the central pore of the protein, is weakly accessible to the solvent and less mobile than W190. Its amplitude of rotation increases upon binding of Ca(2+) and returns to its original value when interacting with membranes. Ca(2+) concentration for half binding of the W5A mutant to negatively charged membranes is approximately 0.5 mM while it increases to approximately 1 mM for the ANX A3 wild type and to approximately 3 mM for the W190 ANX A3 mutant. In addition to the expected perturbation of the W190 environment at the contact surface between the protein and the membrane bilayer, binding of the protein to Ca(2+) and to membranes modulates the flexibility of the ANX A3 hinge region at the opposite of this interface and might affect its membrane permeabilizing properties.

Stabilization of O-pyromellitylgramicidin channels in bilayer lipid membranes through electrostatic interaction with polylysines of different chain lengths

Functioning of membrane proteins, in particular ionic channels, can be modulated by alteration of their arrangement in membranes. We addressed this issue by studying the effect of different chain length polylysines on the kinetics of ionic channels formed in a bilayer lipid membrane (BLM) by O-pyromellitylgramicidin carrying three negative charges at the C-terminus. The method of sensitized photoinactivation was applied to the analysis of the channel association-dissociation kinetics (characterized by the exponential factor of the curve describing the time course of the flash-induced decrease in the transmembrane current, tau). Addition of polylysine to the bathing solutions of BLM led to the deceleration of the photoinactivation kinetics, i.e. to the increase in tau. It was shown here that for a series of polylysines differing in their chain lengths, the value of tau grew as their concentration increased above a threshold level until at a certain concentration of each polylysine tau reached maximum. At higher polylysine concentrations tau began to decrease and finally became close to the control level observed in the absence of polylysine. With lengthening of the polylysine chain the maximum value of tau increased, the concentration dependence became steeper, and the threshold concentration decreased. The increase in the ionic strength of the medium shifted the concentration dependence of tau to higher polylysine concentrations and decreased the maximum value of tau. It was concluded that the increase in tau was caused by the formation of domains of O-pyromellitylgramicidin molecules induced by binding of polylysines. This can be related to functional aspects of polycation-induced sequestering of negatively charged transmembrane peptides in neutral membranes.

S100A1 and S100B interactions with annexins

Members of the annexin protein family interact with members of the S100 protein family thereby forming heterotetramers in which an S100 homodimer crossbridges two copies of the pertinent annexin. Previous work has shown that S100A1 and S100B bind annexin VI in a Ca(2+)-dependent manner and that annexin VI, but not annexin V, blocks the inhibitory effect of S100A1 and S100B on intermediate filament assembly. We show here that both halves of annexin VI (i.e., the N-terminal half or annexin VI-a and the C-terminal half or annexin VI-b) bind individual S100s on unique sites and that annexin VI-b, but not annexin VI-a, blocks the ability of S100A1 and S100B to inhibit intermediate filament assembly. We also show that the C-terminal extension of S100A1 (and, by analogy, S100B), that was previously demonstrated to be critical for S100A1 and S100B binding to several target proteins including intermediate filament subunits, is not part of the S100 surface implicated in the recognition of annexin VI, annexin VI-a, or annexin VI-b. Evaluation of functional properties with a liposome stability and a calcium influx assay reveals the ability of both S100 proteins to permeabilize the membrane bilayer in a similar fashion like annexins. When tested in combinations with different annexin proteins both S100 proteins mostly lead to a decrease in the calcium influx activity although not all annexin/S100 combinations behave in the same manner. Latter observation supports the hypothesis that the S100-annexin interactions differ mechanistically depending on the particular protein partners.

Tethered polymer-supported planar lipid bilayers for reconstitution of integral membrane proteins: silane-polyethyleneglycol-lipid as a cushion and covalent linker

There is increasing interest in supported membranes as models of biological membranes and as a physiological matrix for studying the structure and function of membrane proteins and receptors. A common problem of protein-lipid bilayers that are directly supported on a hydrophilic substrate is nonphysiological interactions of integral membrane proteins with the solid support to the extent that they will not diffuse in the plane of the membrane. To alleviate some of these problems we have developed a new tethered polymer-supported planar lipid bilayer system, which permitted us to reconstitute integral membrane proteins in a laterally mobile form. We have supported lipid bilayers on a newly designed polyethyleneglycol cushion, which provided a soft support and, for increased stability, covalent linkage of the membranes to the supporting quartz or glass substrates. The formation and morphology of the bilayers were followed by total internal reflection and epifluorescence microscopy, and the lateral diffusion of the lipids and proteins in the bilayer was monitored by fluorescence recovery after photobleaching. Uniform bilayers with high lateral lipid diffusion coefficients (0.8-1.2 x 10(-8) cm(2)/s) were observed when the polymer concentration was kept slightly below the mushroom-to-brush transition. Cytochrome b(5) and annexin V were used as first test proteins in this system. When reconstituted in supported bilayers that were directly supported on quartz, both proteins were largely immobile with mobile fractions < 25%. However, two populations of laterally mobile proteins were observed in the polymer-supported bilayers. Approximately 25% of cytochrome b(5) diffused with a diffusion coefficient of approximately 1 x 10(-8) cm(2)/s, and 50-60% diffused with a diffusion coefficient of approximately 2 x 10(-10) cm(2)/s. Similarly, one-third of annexin V diffused with a diffusion coefficient of approximately 3 x 10(-9) cm(2)/s, and two-thirds diffused with a diffusion coefficient of approximately 4 x 10(-10) cm(2)/s. A model for the interaction of these proteins with the underlying polymer is discussed.